The resolution of an optical system, including a projection optical system as used in microlithography, is limited by diffractive limitations of the wavelength of electromagnetic radiation used to make the exposure. In recent years, with the increased miniaturization of semiconductor integrated circuit elements, the limitations of using visible or ultraviolet light for microlithography have become apparent. For improved resolution, microlithography techniques have been developed that use X-rays, electron beams or charged particle beams, such as ion beams, etc. (Herein, beams of electrons or of ions are collectively referred to as "charged particle beams".)
X-rays and charged particle beams have shorter wavelengths than the visible or ultraviolet illumination light conventionally used for microlithography. Thus, X-rays and charged particle beams are potentially able to provide more resolution and thus to project more intricate circuit patterns (i.e., greater circuit density and smaller feature size) onto a wafer than ultraviolet or visible-light microlithography methods.
In order to project a circuit pattern, a mask defining the circuit pattern is required. A representative portion of a conventional mask 41 is shown in FIG. 6, particularly for use with a charged particle beam. The mask 41 comprises a membrane 42, transmissive to the charged particle beam, on which supports 43 are formed. The supports 43 provide the mask 41 with sufficient rigidity to ensure that the mask remains planar during use. The mask 41 is divided into multiple subfields 44 separated from one another by boundary regions 45. The boundary regions are situated opposite the supports 43 on an opposing surface of the membrane 42. Each subfield 44 defines features 46 of the corresponding portion of the overall pattern defined by the mask.
In the mask 41 shown in FIG. 6, the supports 43 are conventionally formed by selectively etching a layer of monocrystalline silicon with a suitable silicon-etching solution (see, e.g., Japan Kokai patent publication no. HEI 2-170410). The supports 43 are typically etched in their height dimension by exploiting a difference in etching rates of the (100) plane versus the (111) plane of the monocrystalline silicon. For example, the wall surface 43a of the support 43 is conventionally the (111) plane of crystalline silicon which is normally tilted at 54.7.degree. relative to the plane of the membrane 42. Etching to form the supports 43 preserves this inclination which leaves the supports 43 wider than required for rigidity and thus undesirably increases the surface area of the mask 41.
Whenever the illumination source for the mask produces divergent rays (such as X-rays), the inclination of the walls 43a can function as a relief for the divergence of the X-rays. In such instances, the width of the supports 43 does not pose a serious problem.
However, masks as described above are not limited to divergent X-rays. i.e., such masks can be used with charged particle beams or with X-rays produced from synchrotron radiation-beam sources. Such illumination methods exhibit extremely small divergence angles. In such instances, inclination of the support walls 43a provides no benefit because the illumination beam perpendicularly illuminates the mask. In addition, the increased mask size from inclining the walls 43a can have a substantial adverse effect, such as necessitating larger optics to accommodate the larger mask field and the need to use mask stages having a wider range of movement.
As shown in FIG. 7, in conventional pattern transfer methods employing charged particle beams, the patterns defined in the subfields 44a-44d on the mask 41 are transferred to corresponding regions 48a-48d on the wafer 47 or other suitable substrate. During transfer of the subfield patterns, the portions of the mask 41 occupied by the boundary regions 45 (and thus by the supports 43 underlying the boundary regions) are not transferred. Rather, the subfields are "stitched" together on the surface of the wafer 47 so that the subfield patterns interconnect with each other on the wafer surface in a manner that produces the desired overall pattern. The subfields 44a-44d on the mask 41 are normally transferred by projection of a demagnified image of the subfields using a suitable optical system (not shown). To eliminate projection of the boundary regions 45 onto the wafer 47, the optical system provides the charged particle beam or X-ray passing through the mask with an additional deflection equal to the width of the boundary region 45. This permits each of the projected subfields on the wafer 47 to be "stitched" together in a manner such that the boundary regions do not appear on the wafer. Unfortunately, an increased deflection distortion can accompany such additional deflection. Because such distortion can have a significant detrimental effect on the transfer accuracy, there is a need for masks in which the boundary regions 45 (and thus the supports 43) are as narrow as possible.